Hello out there. I am currently taking a class in Assembly Language and I have run into some difficulty in an assignment I have. The object is to have a 12 character string display forward then backwards then forwards again. I can get the string to display one way or the other but not both. I am a bit lost as to what I have wrong. My code is as follows:
TITLE Reverse
.model small
.stack 64
.data
tibbar1 db 'USS KITTY HAWK', '$'
tibbar2 db 14 dup ('*'), '$'
.code
main PROC far
mov ax, @data
mov ds, ax
mov es, ax
mov cx, 18h
lea si, tibbar1
lea di, tibbar2
A10:
mov al, [si]
mov [di], al
inc si
inc di
dec cx
jnz A10
A20:
mov al, [si]
mov [di+0dh], al
inc si
dec di
dec cx
jnz A20
mov ah, 06h
mov al, 00h
mov bh, 00bch
mov cx, 0000h
mov dx, 184fh
int 10h
mov ah, 09h
lea dx, tibbar2
int 21h
mov ax, 4c00h
int 21h
MAIN ENDP
END MAIN
I have asked my instructor for help but he teaches as a side gig to his main job so he is not very timely in getting back.
Cheers,
Jay
Maybe this will help some.
; reverse.asm reverse any string entered
;
.model small
.stack 200h
.data
maximum_string_length equ 10
string_to_reverse db maximum_string_length dup (?)
reverse_string db maximum_string_length dup (?)
.code
start:
mov ax,@data
mov ds,ax
mov ah,3fh
mov bx,0
mov cx,maximum_string_length
mov dx,OFFSET string_to_reverse
int 21h
and ax,ax
jz done
mov cx,ax
push cx
mov bx,OFFSET string_to_reverse
mov si,OFFSET reverse_string
add si,cx
dec si
reverseloop:
mov al,[bx]
mov [si],al
inc bx
dec si
loop reverseloop
pop cx
mov ah,40h
mov bx,1
mov dx,OFFSET reverse_string
int 21h
done:
mov ax,4c00h
int 21h
end start
So where would I put the string that needs to be reversed? Is it in this line:
maximum_string_length equ 10
Thank you for your help in this.
Cheers,
Jay
The example I posted waits for the user to input a string. After he hits enter, then it reverses it and shows it on the s screen.
It's been a while since I did any 16 bit coding. Look for Ralf Brown's Interrupt List which tells all the functions
of DOS assembly.
Andy
Ahh ok that is why it was not doing anything when I ran it. We are still early in the term and have not quite got to the point of handeling input from a user yet.
Cheers,
Jay
You can find the interrupt list here.
http://www.cs.cmu.edu/~ralf/files.html
Thanks for the list. It is a bit over my head but Iam sure once Iget a little more comfortable in coding I will be able to use it better. Although I still am unsure what I need to do to get the display I need. Sometimes I really wish I could get a good teacher for once in a programing class.
Cheers,
Jay
Here is a list of assembly resources etc.
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QuoteThe object is to have a 12 character string display forward then backwards then forwards again.
I see nothing in this description that requires you to reverse the string. If you displayed the string a character at a time, using interrupt 21h function 2 for example, you could loop through the characters forwards or backwards to generate the display. You could use any base or index register to access the individual characters, for example:
mov dl, [si]
To progress through the string forwards you could load the base or index register with the offset address of the string (I would use the OFFSET operator) and the counter with the length of the string (I would use the LENGTHOF operator). After each character you could increment the base or index register and decrement the counter, continuing until the counter reached zero. To progress through the string backwards you could load the base or index register with the offset address of the string, plus the length of the string, minus one (I would use an expression of the form "OFFSET thestring + LENGTHOF thestring - 1"). After each character you could decrement the base or index register and decrement the counter, continuing until the counter reached zero.
To move the cursor to the next line you could display a carriage return character (ASCII 13) followed by a line feed character (ASCII 10).
BTW, with this method you would have no need for the trailing '$'.
I feel quite the villiage idiot. I have a general understanding of what you are saying but don't quite know how to type the code to do that. Man I wish I had some one close by that knew what they were doing to teach me.
Ok after a day of banging my head against the wall I still have not figured out what to do. I guess I need someone to hold my hand and walk me through this.
Cheers,
Jay
my instructor finially wrote back but I am not sure what he means exactly. I am so frustrated right now. Any this is what he said:
Quotehi!! ... i went over your attachment ... the first loop is moving string from one place to the other as is, but then for loop 2 you use lea to put address in bx ?? , not in e or d ?? ... this is why it doesn't work ... only si and di handle data addresses ... try now ... aloha!!
Can anyone help me figure out what he means?
Cheers,
Jay
I don't know what your code currently looks like, so this is just a guess. For 16-bit code, where most of the instructions can use any base (BX, BP) or index (SI, DI) register for indirect memory operands, LODS_ uses only SI, STOS_ and SCAS_ only DI, and MOVS_ only SI and DI.
The code that I sent his is
TITLE Reverse
.model small
.stack 64
.data
tibbar1 db 'USS KITTY HAWK', '$'
tibbar2 db 14 dup ('*'), '$'
;tibbar3 db 14 dup ('*'), '$'
.code
main PROC far
;.startup
mov ax, @data
mov ds, ax
mov es, ax
mov cx, 18h
lea si, tibbar1
lea di, tibbar2
A10:
mov al, [si]
mov [di], al
inc si
inc di
dec cx
jnz A10 ;JUMP AND DO THE LOOP AGAIN!!
;mov ah, 06h
;mov al, 00h
;mov bh, 00bch
;mov cx, 0100h
;mov dx, 184fh
;int 10h
;mov ah, 09h
;lea dx, tibbar2
;lea bx, tibbar3
;int 21h
;mov ax, 4c00h
;int 21h
A20:
mov al, [si]
mov [di+0dh], al
inc si
dec di
dec cx
jnz A20 ;JUMP AND DO THE LOOP AGAIN!!
mov ah, 06h
mov al, 00h
mov bh, 00bch
mov cx, 0000h
mov dx, 184fh
int 10h
mov ah, 09h
lea dx, tibbar2
;lea bx, tibbar3
int 21h
mov ax, 4c00h
int 21h
;.exit
MAIN ENDP
END MAIN
I know there is a lot of extra lines but I was trying to mix and match with out having to retype things. I just can't get the basics I guess. :'(
Cheers,
Jay
I could not figure out what you are trying to do with parts of your code, so I modified the first loop to reverse the string as it was copied, and eliminated the buggy second loop.
TITLE Reverse
.model small
.stack 64
.data
tibbar1 db 'USS KITTY HAWK', '$'
tibbar2 db 14 dup ('*'), '$'
;tibbar3 db 14 dup ('*'), '$'
.code
main PROC far
;.startup
mov ax, @data
mov ds, ax
mov es, ax
; REVERSE THE STRING WHILE COPYING IT
;mov cx, 18h ; ?????
MOV CX, 14
lea si, tibbar1
;lea di, tibbar2
; THE "- 2" IS NECESSARY TO AVOID THE '$'
MOV DI, OFFSET tibbar2 + SIZEOF tibbar2 - 2
A10:
mov al, [si]
mov [di], al
inc si
;inc di
DEC DI
dec cx
jnz A10 ;JUMP AND DO THE LOOP AGAIN!!
;mov ah, 06h
;mov al, 00h
;mov bh, 00bch
;mov cx, 0100h
;mov dx, 184fh
;int 10h
;mov ah, 09h
;lea dx, tibbar2
;lea bx, tibbar3
;int 21h
;mov ax, 4c00h
;int 21h
COMMENT | THEN YOU HAVE NO NEED FOR THIS LOOP
A20:
mov al, [si]
mov [di+0dh], al
inc si
dec di
dec cx
jnz A20 ;JUMP AND DO THE LOOP AGAIN!!
|
mov ah, 06h
mov al, 00h
mov bh, 00bch
mov cx, 0000h
mov dx, 184fh
int 10h
mov ah, 09h
lea dx, tibbar2
;lea bx, tibbar3
int 21h
mov ax, 4c00h
int 21h
;.exit
MAIN ENDP
END MAIN
Ok my problem is getting a display of a forward string and a backwards string. It should display
USS KITTY HAWK
KWAH YTTIK USS
I cleaned up the code so the extra lines are gone.
TITLE Reverse
.model small
.stack 64
.data
tibbar1 db 'USS KITTY HAWK', '$'
tibbar2 db 14 dup ('*'), '$'
.code
main PROC far
mov ax, @data
mov ds, ax
mov es, ax
; REVERSE THE STRING WHILE COPYING IT
MOV CX, 14
lea si, tibbar1
;lea di, tibbar2
; THE "- 2" IS NECESSARY TO AVOID THE '$'
MOV DI, OFFSET tibbar2 + SIZEOF tibbar2 - 2
A10:
mov al, [si]
mov [di], al
inc si
;inc di
DEC DI
dec cx
jnz A10 ;JUMP AND DO THE LOOP AGAIN!!
COMMENT | THEN YOU HAVE NO NEED FOR THIS LOOP
A20:
mov al, [si]
mov [di+0dh], al
inc si
dec di
dec cx
jnz A20 ;JUMP AND DO THE LOOP AGAIN!!
|
mov ah, 06h
mov al, 00h
mov bh, 00bch
mov cx, 0000h
mov dx, 184fh
int 10h
mov ah, 09h
lea dx, tibbar2
int 21h
mov ax, 4c00h
int 21h
MAIN ENDP
END MAIN
I tried to have it display the string forward in the second loop by changing:
mov [di+0dh], al
inc si
dec di
to
mov [di], al
inc si
inc di
But that didn't work.
Cheers,
Jay
The modified code I posted correctly reverses the string, and displays the reversed string. The purpose of terminating strings with a '$' is normally so they can be displayed with Interrupt 21h, function 9. The code that displays the reversed string is:
mov ah, 09h
lea dx, tibbar2
int 21h
So code to display the original string would be:
mov ah, 09h
lea dx, tibbar1
int 21h
Ok I figured out why the second loop was not working. I was not familiar with the pipe's function . Now when it displays I get KWAH YY HAWK. So it looks like it is reversing and printing forward but printing it over each other.
Cheers,
Jay
Run through it by hand on paper and you'll see why it's happening - and that you need to swap two characters at the same time (if you have to do it in-place.)
Just thought I would post the code that finially worked.
TITLE try1
.model small
.stack 64
.data
tibbar1 db 'USS KITTY HAWK', '$', 0
tibbar2 db 14 dup ('*'), '$', 0
tibbar3 db 14 dup ('*'), '$', 0
.code
main PROC far
mov ax, @data
mov ds, ax
mov es, ax
; REVERSE THE STRING WHILE COPYING IT
mov cx, 0eh
lea si, tibbar1
lea di, tibbar2
A10:
mov al, [si]
mov [di+0dh], al
inc si
dec di
dec cx
jnz A10 ;JUMP AND DO THE LOOP AGAIN!!
mov ah, 06h
mov al, 00h
mov bh, 00bch
mov cx, 0000h
mov dx, 184fh
int 10h
mov ah, 02h
mov dh, 11
mov dl, 34
int 10h
mov ah, 09h
lea dx, tibbar2
int 21h
mov cx, 0eh
lea si, tibbar2
lea di, tibbar3
A20:
mov al, [si+0dh]
mov [di], al
dec si
inc di
dec cx
jnz A20 ;JUMP AND DO THE LOOP AGAIN!!
mov bh,00h
mov ah, 02h
mov dh, 14
mov dl, 34
int 10h
mov ah, 09h
lea dx, tibbar3
int 21h
mov ax, 4c00h
int 21h
MAIN ENDP
END MAIN
Cheers,
Jay
You can run up and down on the string using a video offset and the direction flag.
(I put an int16 in there to stop it going bonkers)
ZONE S L
org cs:100
;print.reverse.print forever
PRP
nop ;ZRS
mov bp,B800 ;video
mov,es,bp
mov di,722 ;mid screen
mov si,USS ;start of string
Start
mov cx,E
Print
movsb
inc di
loopnz Print
dec si
inc di
inc di
std
mov cx,E
tnirP
movsb
inc di
inc di
inc di
loopnz tnirP
cld
inc si
inc di
inc di
int 16
jmp Start
ret
USS ch10 USS KITTY HAWK
END
______________
Dont unzip it.
Rename the file extension from "zip" to "com", click it to start in DOS and hit any key to print out the output.
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